A preliminary study to identify pathology present in fish in the lower Olifants River following a large crocodile mortality event

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In the Kruger National Park (KNP) the confluence of the Lower Letaba River and the Olifants River, at the Olifants RIver Gorge, is home to one of the densest populations of large Nile crocodile (Crocodylus niloticus) in South Africa. During the autumn and winter of 2008 and 2009, large numbers of adult crocodiles were found dead in this area, coinciding with flooding of the gorge. Some 180 specimens out of a known population of at least 600 died. Autopsies revealed exceptionally fat carcasses with an abnormal hardening of the fat. Subsequent examination of tissue specimens confirmed an inflammation of the fat, typical of pansteatitis. In July 2009, for the first time, a large fish mortality event was observed within the Olifants Gorge. Affected fish were almost exclusively large Clarias gariepinus specimens in the area where the gorge widens into the Massingir dam. Fish carcasses were also observed to be very fat.
This project is based on the assumption that pollution-associated pathology in fish in the Olifants River in the KNP preceded the pansteatitis syndrome that caused the deaths of 180 crocodiles and that certain non-sacrificial pathological indicators may be used to monitor the situation along the river, thereby preventing crocodile mortality in future.
An increasing prevalence of fat necrosis and steatitis was recorded in catfish sampled from the Olifants River gorge in the period 2009 to 2011, and over 60 % of catfish collected during the most recent sampling from the Olifants Gorge were affected with steatitis. Whereas a contributory role of pollution cannot be ruled out, this study points to the likelihood that the increased abundance of phytoplankton feeding fish species, in particular the alien silver carp (Hypophthalmichthys molitrix) in the diet of both catfish and crocodiles in the Olifants and Sabie gorges was the cause of pansteatitis observed in these two species. habitat change in the gorge brought about by raising of the sluices of the lake would have provided both catfish and crocodiles with an excessive intake of silver carp in the short period during peak summer flow when this species migrates into the Olifants River to spawn.
This study has shown the ecological importance and complexity of oxidative stress in a disturbed aquatic environment (increased sediment, and illustrates the risk associated with the presence of alien invasive fish species within the national park. This study has shown that sharptooth catfish are a suitable monitoring species for pansteatitis.